Broadband light sources, for example light sources with full width at half maximum (FWHM) bandwidth of about 5 nm or greater, are well known in the art and are used in a variety of applications. In particular, broadband light sources such as superluminescent diodes (SLDs), rare-earth-doped superluminescent sources (REDSLSs), and light emitting diodes (LEDs) are useful in applications related to interferometry to avoid coherence noise effects.
In one example application of broadband light sources, fiber optic gyroscopes (FOGs) use the interference of light to measure angular velocity, as known in the art. Rotation is sensed in a FOG with a large coil of optical fiber forming a Sagnac interferometer as described for example in H. C. Lefëvre, The Fiber Optic Gyroscope, 2nd Edition, Boston: Artech House (2014). The induced phase shift between the counterpropagating light waves injected in the sensor coil is proportional to the rotation rate. The proportionality constant, called “scale factor,” is given by 2πLD/λc, where L is the length of the fiber coil, D is the diameter of the fiber coil, c is the speed of light in vacuum, and λ is the average, or centroid, wavelength of the light waves propagating in the coil. The centroid wavelength is defined by equation 1:
      λ    _    =            ∫              λ        ⁢                                  ⁢                  P          ⁡                      (            λ            )                          ⁢        d        ⁢                                  ⁢        λ                    ∫                        P          ⁡                      (            λ            )                          ⁢        d        ⁢                                  ⁢        λ            where λ is the wavelength of the spectral components of the light waves, and P(λ) is the optical power as a function of λ, that is, the spectral distribution of the light waves. Hence the accuracy of the gyroscope is limited by the accuracy with which λ of the light source is known. In particular, for FOGs to be useful in certain navigation applications, the λ must be known to an accuracy of 10 parts per million (ppm) or better over a range of ambient temperature ΔT that can span up to 10° C. or more, that is
            1              λ        _              ⁢                  Δ        ⁢                                  ⁢                  λ          _                            Δ        ⁢                                  ⁢        T              =                    1                  λ          _                    ⁢      α        <                  10                  -          6                    ⁢              1                  °          ⁢                                          ⁢                      C            .                              where the thermal sensitivity of the centroid wavelength is defined as α≡λ/ΔT.
Broadband light sources are particularly advantageous for introducing the light into the sensor coil because phase coherent noise effects due to backscattering noise and polarization coupling is suppressed, the residual intensity noise (RIN) of the FOG decreases with increasing bandwidth, and the zero-rotation drift induced through the Kerr effect by relative variations in the two counterpropagating optical powers is reduced. Such effects would otherwise cause significant reduction in rotation sensitivity and accuracy. The relatively small size, low power consumption and low cost of SLDs are advantageous for many FOG applications. However, the inherent thermal sensitivity of the centroid wavelength αSOURCE of SLDs is typically +250 to +400 ppm/° C., which is problematic for certain FOG applications even when thermoelectric cooling devices and other temperature compensation components, circuits and techniques are utilized. Consequently, REDSLSs, such as erbium-doped fiber amplifiers, having significantly lower centroid wavelength thermal sensitivity, have tended to find application in FOGs. For example, in D. C. Hall et al., “High-stability Er3−-doped superfluorescent fiber sources,” J. Lightwave Tech., Vol. 13, No. 7, pp. 1452-1460, July 1995, a centroid wavelength thermal sensitivity of 3-5 ppm/° C. is reported for an erbium-doped fiber amplifier type REDSLS.
In addition to FOGs, other optical sensors and measuring devices as known in the art, such as accelerometers, pressure sensors, strain sensors, temperature sensors, profilometers, fiber optic link test equipment, and optical coherence tomography systems, provide applications for which broadband light sensors enjoy utility and wherein the accuracy of the centroid wavelength is critical to performance. Various strategies for wavelength stabilization against environmental factors, such as ambient temperature, have been invoked to improve centroid wavelength accuracy. These strategies include both active and passive stabilization methods.